[llvm] [NFC] Fix #106873 - update assignment tracking docs (PR #106959)

[Orlando Cazalet-Hyams via llvm-commits]

OCHyams wrote:

> The TODO list still says it's an "experimental work in progress", which reads a bit strangely for something that's now enabled by default. Perhaps reword that bit as well?

Agreed, done

> I assume the rest of the TODO list is still valid, since you haven't removed any items here...?

Unfortunately so. Actually, while we're here I've added an item.

https://github.com/llvm/llvm-project/pull/106959